A study on the reproducibility by computed tomography (CT) using a phantom

In general, computed tomography (CT) scanning methods for CT image acquisition include axial scanning methods, helical scanning methods and cine scanning methods. Many studies have reported the usefulness of four-dimensional computed tomography (4D CT) scanning for the planning of four-dimensional radiation therapy. On the contrary, no study has reported the reproducibility of 4D CT images for the above-mentioned scanning methods. Therefore, in this study, a phantom that could be moved independently was manufactured, and this manufactured phantom was applied in the same way as in the 4D CT method. Scanning was then performed using the three methods so that the reproducibility of the image could be compared and analyzed. The study used a self-manufactured drive phantom and a respiration-gated drive motor to reproduce movements in the upward/downward directions and in the top/bottom directions. Markers were placed on the drive motor, and a radiopaque material (localizer) was attached to allow positional marking on the marker for the differentiation of movement when performing CT. The top/bottom movement when using the self-manufactured drive phantom was fixed at 1.3 cm (16 times/1 min), and the upward/downward movement was fixed at 0.2 cm (8 rot/1 min) by using a respiration-gated drive motor. After each movement had been fixed, images were taken with axial, helical and cine scanning for the three methods of CT. A radio-opaque material attached to the marker to indicate movement was compared and analyzed by using radiation treatment planning (RTP) equipment to ensure volume reproducibility. The study results revealed that the total volume of the marker was 88.2 cm(3), and when a 3-cm shift in the top/bottom directions and a 0.3-cm shift in the upward/downward directions were taken into account, the total volume was 184.3 cm(3). By CT scan type, the total volume was 135 cm(3) for axial scanning, 164.9 cm(3) for helical scanning and 181.7 cm(3) for cine scanning. Regarding the reproducibility of movement, the cine scan depicted the marker much better than the other scans did. In addition, the reproducibility of the radiopaque material attached to the marker was better for the cine scan.